Subsurface safety valves are typically hydraulically controlled from a remote location using one or two control lines. An advantage of a two control line system is that hydrostatic pressure in each line is canceled out so that a closure spring for a flow tube does not need to resist hydrostatic pressure as is the case with single control line systems. In two line control systems pressure on top of an operating piston moves a flow tube against a flapper to open the valve. Removal of such pressure from the main control line allows a closure spring to reverse movement of the flow tube to allow the flapper to rotate 90 degrees to closed position of the safety valve. In the past operators have wanted or regulations required a barrier in the second or balance control line so that if tubing pressure leaks into the hydraulic system there would be a barrier to keep hydrocarbons from reaching a surface location through the balance line.
The floating piston in the balance line served this purpose as a barrier. In normal valve operations pressure applied in the main control line to the top of a piston whose movement shifted the flow tube would result in hydraulic fluid displacement to the underside of the floating piston. Conversely, as pressure was removed from the main control line and the closure spring pushed up the flow tube hydraulic fluid would be drawn into the safety valve from under the floating piston to enable the safety valve to close. The floating piston would just move up when the safety valve open and reverse its motion when the safety valve closed, each time displacing an equal volume of hydraulic fluid as movement of the operating piston had displaced. The floating piston was sometimes biased toward the down position to put it in the ready position for safety valve opening.
Sometimes, seals could leak in such safety valve hydraulic systems such that the much higher tubing pressure could leak into the balance control line and against the underside of the floating piston. This could happen slowly taking months or even years to reach an extreme condition where the floating piston would be up against an upper travel stop with tubing pressure under it. As a result the safety would not be functional to open since the operating piston in the safety valve could not displace hydraulic fluid because the floating piston could not move because it was forced against an upward travel stop due to tubing pressure leaking past a seal. When this happened in the past the safety valve would need to be removed, which caused very expensive downtime.
The present invention is a reconfiguration of the two control line system that incorporates the floating piston working normally the same way as it worked in the past. What is different is the addition of an operable one way valve that can be opened with pressure applied to the balance line such that when such equalizing valve was forced open from the balance line applied pressure, the pressure on opposed sides of the floating piston could equalize and the position of the floating piston could change. The floating piston, now placed in pressure balance on its opposed ends could be biased away from its upper travel stop. Doing this would again make the safety valve operable to open as the hydraulic system would no longer be liquid locked by virtue of the floating piston sitting against its upper travel stop under tubing pressure. In essence the balance line pressure would be raised to the level of the tubing pressure or less depending on seal geometries to get the equalizer valve to open to allow a return spring acting on the floating piston to bias it back to a lower travel stop to allow reopening of the valve without well shutdown and safety valve removal. Many times the seal leakage is so slow that the ability to reposition the floating piston can allow many more years of service for the safety valve. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims. The following references are illustrative of control systems used in the past for safety valves in a borehole application: U.S. Pat. Nos. 5,906,220; 7,743,833; 8,534,317 and US 2008/0314599.